Shorting a stock places a bet that its price will fall. You short by promising to hand over the stock, on such and such date for $X. The gamble pays off if the price drops below $X.
But, to sell short, you can't own the stock. You are "short" it. How do you legally sell a stock you don't own? Under Securities and Exchange Commission rules, you have to "borrow" it. Brokers lend stock for a fee.
The short-selling plaintiffs in Electronic Trading Group LLC v. Banc of America Securities LLC (In re Short Sale Antitrust Litig.), No. 08-0420-cv (2d Cir. Dec. 3, 2009), sued brokers under section 1 of the Sherman Act. They alleged that the brokers conspired to drive up rent on stocks they loaned. The district court granted defendants' motion to dismiss under Credit Suisse (USA) LLC v. Billing , 551 U.S. 264 (2007), in which the Court held that SEC regulation of initial public securities offerings implicitly preempted antitrust law. The Second Circuit affirmed on the same ground.